The present invention pertains to the brightening of chemical pulps and deinked, mixed office waste pulps, and particularly to the brightening of such pulps in a peroxide or peroxide/oxygen stage of brightening.
The process of making paper from wood involves the following general stages: (1) bark removal; (2) wood chipping; (3) pulping; (4) brightening; and (5) forming a sheet of paper on a machine. During pulping, wood is reduced to its fibrous state, with a portion of the lignin content in the wood being removed. Pulps can be divided into two main categoriesxe2x80x94chemical and mechanical pulpsxe2x80x94depending on how they are made from wood. Chemical pulping involves the use of chemical reagents to effect a separation of the cellulose fibers from the other wood components, such as lignin and other extraneous compounds. In the process, most of the hemicelluloses are also dissolved. Thus, the yield for chemical pulping is typically 40-50% on wood.
Mechanical pulping involves the reduction of wood to the fibrous state by mechanical means, such as by grinding logs into pulp by large revolving grindstones. These pulps are called xe2x80x9cmechanicalxe2x80x9d because a significant amount of mechanical energy (grinding or refining) is required to break down the wood chips. Except for a few water soluble components, all of the constituents of the wood are present in the ground wood pulp. Thus, mechanical pulps are characterized by their high yield and high lignin content. For example, although chemical pulps contain only about 5% lignin (weight basis on pulp) after pulping, mechanical pulps typically contain greater than 20% lignin for hardwoods and 25% for softwoods after pulping.
In general, the brightening of chemical and mechanical pulps occur by different mechanisms. This difference in approach is due, in part, to the difference in lignin content between the chemical and mechanical pulps and to the different nature of the lignin in chemical pulps than that in mechanical pulps. The remaining lignin in chemical pulps is typically more difficult to degrade than the majority of the lignin remaining in mechanical pulps. For example, chemical pulps, such as kraft pulps, are more difficult to brighten by H2O2. Thus, instead of the 60xc2x0 C. bleaching temperature used for mechanical pulps, values in the range of 110xc2x0 C. are used for kraft pulps (which use sodium hydroxide and sodium sulfide as the primary chemical reagents). As another example of the differences in brightening the two different types of pulp, chlorine dioxide (ClO2), a stable free radical, is involved in the brightening of more than 90% of bleached kraft pulp produced in the U.S.A. per year. On the other hand, it has been reported that chlorine dioxide actually darkens mechanical pulps. In sum, results obtained with mechanical pulps cannot be considered as being predictive for chemical pulps.
Various additives have been proposed to improve the brightening process of mechanical pulps or mixed chemical/mechanical pulps having a high lignin content (i.e., above 20% by weight on pulp). Sodium silicate is widely used for hydrogen peroxide brightening of such pulps. It acts as a peroxide stabilizer and as a buffer during the bleaching reaction. Sodium silicate, in combination with a small dose of MgSO4, has long been known to improve the brightness of mechanical pulps. The two chemicals are known to form an intermediate that adsorbs or complexes transition metal species which would otherwise undesirably catalyze the decomposition of H2O2. The two chemical combination is widely used in installations that bleach mechanical pulps. When magnesium sulfate has been used for such pulps, however, no increase in brightness has been observed when the MgSO4.7H2O dose is increased above 0.05% on mechanical pulp (2.0 mmoles/kg). Accordingly, previous researchers had no need to add more magnesium to their pulps.
Turning to the brightening of chemical pulps, some results have suggested that silicate has a negative effect on H2O2 bleaching of kraft pulps. In some cases, the inclusion of silicate lowered the brightness by 4.6 percentage points (measured as % GE brightness in accordance with TAPPI Standard T452 om-92) and by 2-7 percentage points.
In peroxide brightening of mechanical pulps, the key is nucleophilic degradation of carbonyl compounds in the native lignin without the oxidation of phenols to o- and p-quinones, as shown below in reactions [A] and [B], respectively. 
Reaction [A] is the desirable degradation of carbonyl compounds by the perhydroxyl anion, OOHxe2x88x92. This reaction breaks down the carbonyl compounds, which absorb light in the visible range, to a more soluble form to be washed away by water. On the other hand, reaction [B] represents the undesirable formation of o- and p-quinones, which are less soluble in water and also absorb light in the visible range.
The perhydroxyl anion is generated by the dissociation of H2O2 as shown by equation [1] below. Mild conditions have to be used to prevent the oxidation of phenolics.
H2O2+H2OH3O++OOHxe2x88x92 pKa=11.6xe2x80x83xe2x80x83[1]
Mechanical pulps contain high concentrations of lignin and extractives whose negatively charged sites may complex transition metals. Also, some transition metal catalysis can be tolerated because lignin is a very good radical scavenger. It will scavenge the superoxide anion (.O2xe2x88x92) and prevent wasteful decomposition to O2 (equation [2]).
.O2xe2x88x92+M(m+1)+xe2x86x92O2+Mn+xe2x80x83xe2x80x83[2]
Actually, .O2xe2x88x92 is nucleophilic and its reaction with lignin is reported to result in increased brightness. In sum, transition metal deactivation by magnesium silicates is important but not critical to the brightening of mechanical pulps.
Unlike softwood mechanical pulps which typically contain about 25% lignin (by weight on pulp), chemical pulps enter the final brightening stage with typically less than 2% lignin, which is typically colored and very difficult to oxidize. The approach with H2O2 is nucleophilic degradation by OOHxe2x88x92. However, equation [2] becomes favorable because there is not a large amount of reactive lignin to scavenge .O2xe2x88x92. Equation [2], in conjunction with equations [3] and [4], results in wasteful decomposition of H2O2.
Mn++H2O2xe2x86x92M(m+1)++.OH+xe2x88x92OHxe2x80x83xe2x80x83[3]
.OH+xe2x88x92OOHxe2x86x92.O2xe2x88x92+H2Oxe2x80x83xe2x80x83[4]
Equation [3] is favorable for Cu(I) and Fe(II) but not for Mn(II). A more probable mechanism for Mn(II) is outlined below. The subscripted s indicates soluble Mn(IV) and Mn(III).
Mn2+(s)+H2O2xe2x86x92Mn4+(s)+2OHxe2x88x92
Mn4+(s)+xe2x88x92OOHxe2x86x92Mn3+(s)+H++.O2xe2x88x92
Mn3+(s)+xe2x88x92OOHxe2x86x92Mn2+(s)+H++.O2xe2x88x92
Lignin degradation by OOHxe2x88x92 requires a high temperature because of the unreactive nature of the lignin in chemical pulps. At 110xc2x0 C., transition metal deactivation is extremely important.
With respect to O2 delignification, a simple and well-accepted scheme for free radical generation is provided below. RH is a reactive structure in the solution phase.
RH+O2xe2x86x92R.+.OOHxe2x80x83xe2x80x83[5]
.OOH+H.xe2x86x92HOOH (H2O2)xe2x80x83xe2x80x83[6]
(Abstraction of H atom from lignin)
R.+O2xe2x86x92ROO.xe2x80x83xe2x80x83[7]
ROO.+H.xe2x86x92ROOHxe2x80x83xe2x80x83[8]
H2O2 and ROOH will be affected by transition metals in the same manner as in a peroxide/oxygen stage. Thus, the oxygen stage is similar to the peroxide/oxygen stage in that they both involve heterolytic (i.e., ionic) reactions and homolytic (i.e., free radical) reactions. In addition, they are both similar in that both rely on hydrogen peroxide, either as added in the peroxide/oxygen stage or as generated in reaction [6] above in the oxygen stage, to perform a brightening function. It is assumed that organic peroxides (ROOH in equation [8]) generated during oxygen delignification dissociate to form ROOxe2x88x92 nucleophiles that brighten lignin. Accordingly, in both stages (as well as in a peroxide stage), one must be concerned about transition metals undesirably catalyzing free radical reactions.
The present invention provides a combination of additives and methods for brightening pulps containing less than 18% lignin. The invention can be used in either the peroxide, oxygen, or peroxide/oxygen brightening stages. The additives according to the present invention include an aqueous sodium silicate solution, an alkali agent, and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. The alkali agent is added in an amount sufficient to maintain a pH of at least about 8, and the magnesium compound is added in an amount to achieve, along with any other dissociated magnesium, an Mg:SiO2 mass ratio of between about 1:46 to about 1:2.
A method for brightening pulp according to the present invention involves mixing pulp containing less than 18% lignin with an aqueous sodium silicate solution, an alkali agent, and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. The alkali agent and magnesium compound are added to achieve the same pH range and weight ratio mentioned above. The method also includes heating the mixture to allow the mixture to react to cause a portion of the lignin in the pulp to degrade.
According to another embodiment of the present invention directed to the peroxide or peroxide/oxygen brightening stages, a method for brightening pulp containing transition metals, hydrogen peroxide, and less than 18% lignin includes first forming a sodium silicate mixture having a high percentage of high molecular weight silicates. This is accomplished by mixing sodium silicate and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. This sodium silicate mixture is then added to the pulp to adsorb at least a portion of the transition metals.
The present invention also includes an aqueous composition for use in brightening pulps. The composition includes pulp containing less than 18% lignin, an aqueous sodium silicate solution, an alkali agent, and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. The alkali agent and magnesium compound are added to achieve the same pH range and weight ratio mentioned above.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.
The present invention is directed to the brightening of certain pulps during peroxide, oxygen, or peroxide/oxygen brightening. As used herein, the term xe2x80x9cbrightening stagexe2x80x9d shall mean either a peroxide stage, an oxygen stage, or a peroxide/oxygen stage. The typical operating conditions and conventional additives to these stages are well known. During such brightening, hydrogen peroxide serves to degrade carbonyl groups of lignin, rendering them more water soluble. In the absence of such degradation, lignin would remain in the pulp, causing the brightness of the pulp to remain lower than would be if such lignin were removed. Transition metals present in the pulp undesirably react with hydrogen peroxide thereby precluding hydrogen peroxide from serving its function of degrading lignin. The present invention is based in part on the recognition that higher molecular weight silicates assist in adsorbing transition metals thus preventing them from undesirably reacting with hydrogen peroxide. The present invention is also based on the recognition that Mg(OH)+ plays a significant role in the molecular weight distribution of a sodium silicate solution. More particularly, the presence of Mg(OH)+ in a sodium silicate solution, over a certain Mg:SiO2 mass ratio, enhances the formation of higher molecular weight silicates.
Accordingly, the present invention is directed to various compositions and methods embodying this concept. The invention provides a combination of additives (either to hydrogen peroxide, oxygen, or a combination thereof, depending on which one of the three brightening stages is being employed) for brightening pulps containing less than 18% lignin. In this case, these additives include an aqueous sodium silicate solution, an alkali agent, and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. The invention also encompasses an aqueous composition for use in brightening pulps comprising pulp containing less than 18% lignin, hydrogen peroxide (either as an additive or as reacted from reaction [6] above), an aqueous sodium silicate solution, an alkali agent, and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. The present invention also provides a method for brightening pulp including forming a mixture of the pulp, hydrogen peroxide, aqueous sodium silicate solution, alkali agent, and magnesium compound, then heating the mixture to allow it to react to cause a portion of the lignin to degrade. Finally, the invention is characterized as a method for brightening pulp containing transition metals and less than 18% lignin. This method includes the steps of forming a sodium silicate mixture having a high percentage of high molecular weight silicates by mixing sodium silicate and a suitable magnesium compound, and adding the sodium silicate mixture to the pulp.
The pulp used in connection with the present invention is defined broadly as any pulp having less than 18% lignin (on pulp). Unless otherwise specified, all percentages of lignin include all forms of lignin and are given as weight percentages of the total pulp, on a dry weight basis, so that 18% lignin means that there are 18 grams of lignin in a 100 gram sample of pulp (excluding the weight of water).
Stated another way, the invention can be used with any pulp in which more than 50% of the lignin (by weight, on wood) is removed during pulping. This means that more than half of the original lignin present in the wood is removed during the pulping process. For softwoods, having an average lignin content of 28.6% (by weight, on wood), the removal of half of the original lignin content of the wood during a conventional chemical pulping process results in a pulp having 23.3% lignin (on pulp). For hardwoods, having an average lignin content of 24.1% (by weight, on wood), the removal of half of the original lignin content of the wood during a conventional chemical pulping process results in a pulp having 18.0% lignin (on pulp). The development of these final lignin contents factors in the yield from conventional chemical pulping processes. Thus, any pulp with less than 18% lignin (on pulp) would typically include at least some pulp which was processed by chemical pulping or was derived from a wood having a relatively low initial content of lignin.
In general, chemical pulping processes are continued until the lignin content is reduced to about 5% (on pulp) or less. It is preferable to use the present invention to brighten such pulps. In addition, chemical pulps typically undergo further delignification before the final brightening, stage (i.e., peroxide or peroxide/oxygen) such that the pulps contain about 1-2% lignin before the final brightening stage. Pulps brightened by the present invention can also include such pulps which have been further delignified. The present invention can also be used to brighten pulps having recycled pulps, in which chemical and mechanical pulps are mixed, as long as the average lignin content is less than about 18%. Pulps brightened by the present invention also include semichemical pulps with lignin content varying from approximately 10 to 18%. In Example 4 below, bleached and unbleached samples of de-inked mixed office waste consisting of 65% sorted white ledger and 35% office pack were obtained from a mill in the American Midwest.
The additives of the present invention are used in the peroxide, oxygen, or peroxide/oxygen stages of brightening. The process parameters of these stages are well known. For example, a typical charge of peroxide loading is 20 kg of hydrogen peroxide per metric ton of pulp. The hydrogen peroxide is mixed while the mixture is heated, typically to a temperature of between about 90xc2x0 C. to about 130xc2x0 C., often 110xc2x0 C. Water is also added, typically to achieve a consistency of about 15% solids. Of course, reaction time, temperature, and consistency are all interrelated and can be varied in a known manner. A hydrogen peroxide/oxygen stage is similar to a hydrogen peroxide stage except that the slurry is pressed with oxygen.
An oxygen stage is similar to a hydrogen peroxide/oxygen stage except that no hydrogen peroxide is added. Oxygen delignification is also conducted over a wider consistency range (10% to 35% solids) but 12% to 15% solids is typical. In general, an oxygen stage does not brighten as well as a peroxide stage or as a peroxide/oxygen stage. Oxygen or an oxygen-containing gas (such as air) can be used to pressurize the mixture, and the partial pressure of oxygen typically ranges between 0.38 MPa and 1.48 MPa.
An additive of the present invention is an aqueous sodium silicate solution. Any commercially available aqueous sodium silicate solution may be used, although it is preferable to use as pure of a sodium silicate solution as is practically possible. The sodium silicate solution may be added in an amount to achieve a concentration of from about 0.14% to about 1.4% SiO2 on pulp. Preferably, the sodium silicate solution is added in an amount to achieve a concentration of from about 0.28% to about 1.12% SiO2 on pulp.
Another additive according to the present invention is a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. The magnesium compound should be added in an amount to achieve, along with any other dissociated magnesium, an Mg:SiO2 mass ratio of between about 1:46 to about 1:2. Preferably, the Mg:SiO2 mass ratio is between about 1:15 to about 1:3. Most preferably, the Mg:SiO2 mass ratio is between about 1:10 to about 1:3. The mass ratio is of the total magnesium available for formation into Mg(OH)+, including any of the magnesium added and any magnesium inherent in the pulp and existing in a form which readily dissociates and forms Mg(OH)+ over a pH range of at least 8:
The magnesium compound is added in an amount to achieve an actual concentration of from about 0.01% to about 0.2% Mg on pulp (again including any other source of available magnesium). More preferably, the magnesium compound is added in an amount to achieve a concentration of from about 0.02% to about 0.2% Mg on pulp. Preferably, the magnesium compound is magnesium sulfate, added as MgSO4 or MgSO4.7H2. Alternatively, the magnesium compound is MgO, MgCl2, Mg(OH)2 and/or MgNO3, among others. Any number of magnesium compounds can be used, as long as they readily form Mg(OH)+ at pH values of at least 8 and are not detrimental to process equipment.
A critical step in the present invention is the formation of the hydrolyzed magnesium cation Mg(OH)+. It is believed that this cation neutralizes dissociated silanol groups and causes silicate polymerization, as summarized by the equations written below: 
Another additive of the present invention is an alkali agent added in an amount sufficient to maintain a pH of the solution of at least about 8. At pH values of at least about 8, magnesium exists as Mg(OH)+ in an amount sufficient to initiate reaction [9] above. More preferably, the alkali agent is selected and added in an amount sufficient to maintain the pH to a range of between 8 and 12. The alkali agent may be NaOH, Na2O, MgO, Mg(OH)2, K2O, KOH, CaO, and/or Ca(OH)2. Preferably, the non-calcium containing compounds are selected to minimize the formation of silicate scales. Such silicate scales can be detrimental to the quality of the paper product and must be removed periodically resulting in costly stoppages of the process. Because MgO and Mg(OH)2 can serve two functions, namely that as the magnesium compound which readily forms Mg(OH)+ and as an alkali agent, then the magnesium compound and alkali agent can be combined as one additive in some cases.
According to a method of the present invention, the pulp is brightened by mixing the pulp with hydrogen peroxide, an aqueous sodium silicate solution, an alkai agent added in an amount sufficient to maintain the pH of the solution at least about 8, and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations, wherein the magnesium compound is added in an amount to achieve, along with any other dissociated magnesium, an Mg:SiO2 mass ratio of between about 1:15 to about 1:3. This mixture is then heated, which allows the mixture to react to cause a portion of the lignin to degrade (i.e., to break down into a more soluble form to be washed away). In the case where a hydrogen peroxide/oxygen stage is used, the mixture is also pressurized with oxygen, in a known manner. While continuously heated, the chemicals are mixed into the pulp slurry which is retained at an elevated temperature for a period of time sufficient to allow the degradation reactions of the lignin to occur. This typically is between 30 minutes and four hours, depending on the temperature and concentration of the additives in the pulp. Typically, the mixture is heated to a temperature of between about 90xc2x0 C. and about 130xc2x0 C.
As mentioned above, the present invention relies on the formation of high molecular weight silicates to adsorb transition metals to avoid the decomposition of hydrogen peroxide by transition metals. Accordingly, the present invention can be characterized as a method for brightening pulp by forming a sodium silicate solution having a high percentage of high molecular weight silicates by mixing sodium silicate and a magnesium compound which dissociates in the solution to form Mg(OH)+ cations. Then, the sodium silicate solution is added to the pulp to adsorb at least a portion of the transition metals of the pulp. Any increased amount of higher molecular weight silicates appears to improve the adsorption of transition metals, but it is preferable that the constituents are added such that the sodium silicate solution has at least 25% of the silicates with a molecular weight of at least 10,000 daltons.
It is not believed that the order of addition is important to the functioning of the present invention. Typically, the magnesium compound is Epsom salt (MgSO4.7H2O) and is either first dissolved in water or dissolved directly in a commercial aqueous sodium silicate solution. Then the alkali agent(s) is added followed by H2O2. The combination is then mixed into the pulp slurry.